Fluid-film journal bearings provide the primary lateral support for horizontal and vertical turbomachinery rotors. Bearing design is dependent upon criteria such as load, speed, stability, rotor dynamics, lubricants and cost. Within the array of design parameters, preload is significant in controlling bearing performance, which ultimately impacts maintenance and operating costs. Bearing preload is often used to adjust bearing coefficients in order to obtain specific rotor response. Tilt Pad Bearing Preload, Robert C. EisenMann Sr., Sulzer Tech. Rev. (2004) discusses aspects of tilt-pad bearing design and preload where the shaft is oriented horizontally. When the shaft is oriented horizontally, the weight of the shaft assembly, or rotor, provides a well-defined downward force, which pulls the shaft towards the bottom of the bearing clearance. This displacement gives a defined preload as described by the equation:
  Preload  =      1    -                            C          brg                          C          pad                    .      The dynamic response from the bearings can be assumed to be linear, which leads to predictable dynamic behavior in horizontally oriented rotating machines. However, in rotating machines where the shaft is oriented vertically, the weight of the shaft/rotor assembly does not provide significant bearing preload force. The bearings are only lightly loaded and there is no defined direction of the static load. Dynamic responses to fluid interacting with the pump components may include large undesirable amplitudes and/or be non-linear, unstable and difficult to predict.